Variable-speed belt gear

ABSTRACT

A belt gear wherein a first two-step pulley receives torque from a motor and drives a second two-step pulley by way of an endless belt. The mechanism for changing the speed of the second pulley has a selector knob which is rotatable between first and second angular positions and is coaxial with a shifter lever having a fork which straddles a portion of the belt between the two pulleys. One leg of a torsion spring is coupled to an eccentric pin of the knob and the other leg of the torsion spring is coupled to the shifter lever. The spring biases the knob to the second angular position as soon as the knob is rotated, either by hand or by remote control, beyond an intermediate dead-center position on its way from the first angular position, and vice versa. In the first angular position of the knob, the spring biases the shifter lever toward a position in which the lever tends to maintain the belt in engagement with the larger rim portion of one of the pulleys and the smaller rim portion of the other pulley. In the second angular position of the knob, the spring tends to move the shifter lever to a position in which the belt is trained over the smaller rim portion of the one pulley and the larger rim portion of the other pulley. The spring is capable of shifting the belt by way of the lever only when the belt is driven by the first pulley.

United States Patent 1191 Krumbein Feb. 5, 1974 VARIABLE-SPEED BELT GEARFritz Krumbein, Stuttgart-Mohringen, Germany [73] Assignee: Robert BoschPhotokino Gmbll,

Stuttgart, Germany [22] Filed: June 28, 1972 [21] Appl. No.: 267,173

[75] Inventor:

[30] Foreign Application Priority Data Primary Examiner-Charles J. MyhreAssistant ExaminerR. H. Lazarus Attorney, Agent, or Firm-Michael S.Striker ABSTRACT A belt gear wherein a first two-step pulley receivestorque from a motor and drives a second two-step pulley by way of anendless belt. The mechanism for changing the speed of the second pulleyhas a selector knob which is rotatable between first and second angularpositions and is coaxial with a shifter lever having a fork whichstraddles a portion of the belt between the two pulleys. One leg of atorsion spring is coupled to an eccentric pin of the knob and the otherleg of the torsion spring is coupled to the shifter lever. The springbiases the knob to the second angular position as soon as the knob isrotated, either by hand or by remote control, beyond an intermediatedeadcenter position on its way from the first angular position, and viceversa. In the first angular position of the knob, the spring biases theshifter lever toward a position in which the lever tends to maintain thebelt in engagement with the larger rim portion of one of the pulleys andthe smaller rim portion of the other pulley. In the second angularposition of the knob, the spring tends to move the shifter lever to aposition in which the belt is trained over the smaller rim portion ofthe one pulley and the larger rim portion of the other pulley. Thespring is capable of shifting the belt by way of the lever only when thebelt is driven by the first pulley.

11 Claims, 3 Drawing Figures SHEET 1 BF 3 PATENTED FEB 5 I 7 PATENTEDFEB5mm 3,789,685-

SHEET 2 BF 3 Fig.2

PATENTEUFEB 51974 3.789.885

SHEET 30F 3 Fig. 3

f I I 7a Ll 29 I v 22 18 VARIABLE-SPEED BELT GEAR BACKGROUND OF THEINVENTION The present invention relates to improvements in belt gears ingeneral, and more particularly to improvements in belt gears which canbe utilized in photographic apparatus including cameras, projectors orthe like. Still more particularly, the invention relates to improvementsin belt gears which are especially suited for use in motion pictureprojectors (for example, to drive a reel or spool for motion picturefilm at several speeds) and which utilize spring-biased shifters forendless belts, cords, cables, ropes or analogous motion transmittingelements (hereinafter called belts).

Many presently known belt gears are constructed in such a way that thebelt can be shifted from a first pair of rims onto a second pair of rimsor vice versa only when the pulleys around which the belt is trained arein motion. The shifter is normally movable directly by hand and includesa fork whose prongs straddle a portion of the belt and are movabletransversely of the direction of lengthwise movement of such beltportion. The just described belt gears do not allow for requisitecontrol of forces which are transmitted by way of the manually movableshifter.

Certain recent types of belt gears employ springbiased shifters for thebelt and selector means for indirectly changing the position of theshifter by way of the biasing means. An advantage of such belt gears isthat the selector means can be moved to a desired position to therebyselect the transmission ratio between the driving and driven pulleyswhile the belt is idle whereby the biasing means compels the shifter tochange its position as soon as the belt is set in motion. The biasingmeans stores energy when the selector means is adjusted while the beltis idle and is capable of dissipating such energy when the belt isdriven because the belt then offers a greatly reduced resistance tosidewise movement. Furthermore, the biasing means protects the beltagainst excessive stretching not only when the belt is idle but alsowhen the pulleys are in motion.

As a rule, the just described recent types of belt gears employ severaldiscrete springs which are stressed to produce opposing forces acting onthe shifter. An additional spring is used to serve as a yieldable detentfor the selector means so that the latter can remain in a desiredposition. Furthermore, such belt gears employ many additional componentparts which contribute considerably to the initial and maintenance costand render the belt gears prone to malfunction. The belt gears occupytoo much room so that they cannot be used in compact motion pictureprojectors or analogous apparatus.

SUMMARY OF THE INVENTION An object of the invention is to provide asimple, lightweight, rugged and long-lasting variable-speed belt gearwhich is small'enough to be capable of being used in a compact motionpicture projector or the like.

Another object of the invention is to provide a belt gear which candrive one of its pulleys at two or more speeds, wherein the shifter canbe set up to change the speed of the one pulley while the belt is idlebut which allows for immediate speed change when the belt is in motion,and which is assembledof a very small number of simple parts.

A further object of the invention is to provide a variable-speed beltgear wherein the biasing means which is interposed between the selectormeans and the shifter can also serve as a yieldable detent for theselector means.

Still another object of the invention is to provide a variable-speedbelt gear wherein a single resilient element suffices to transmit motionto the shifter and to serve as a simple but reliable detent for theselector means.

An additional object of the invention is to provide a belt gear whosecomponents can be assembled prior to mounting in the housing of a motionpicture projector or the like, wherein the belt is protected againstexcessive stressing in response to movements or biasing of the shifter,and wherein the shifter can indirectly receive motion from manuallyactuatable or remotely controlled selector means.

The improved variable-speed belt gear comprises a driving pulley and adriven pulley each having coaxial first and second rim portions, andendless flexible belt which is trained over the pulleys, and novel speedchanging means which is operable to move the belt from the first rimportions onto the second rim portions or vice versa. The speed changingmeans comprises a speed selector (preferably a rotary knob) which ismovable between first and second positions and through an intermediateposition, a belt-engaging shifter which is movable between a firstposition in which the belt is trained over the first rim portions and asecond position in which the belt is trained over the second rimportions, and resilient means (preferably a simple torsion spring) forrespectively urging the selector and the shifter to their first andsecond positions in the first and second positions of the selector. Theshifter includes a fork, a sheave or another suitable belt-engagingportion which is movable transversely of the direction of movement ofthe adjacent belt portion, and the intermediate position of the selectoris preferably a dead center position. The resilient means is arranged tostore energy in response to movement of the selector to its intermediateposition and to thereupon propel the selector to its first position assoon as the selector moves beyond the intermediate position on its wayfrom the second position, or vice versa.

The tension of the belt is preferably such that, when the pulleys areidle, the shifter remains in its momentary position in spite of movementof the selector whereby the resilient means stores energy which sufficesto move the shifter to a different position as soon as the belt is setin motion in response to rotation of the pulleys.

The novel features which are considered as characteristic of theinvention are set forth in particular in the appended claims. Theimproved belt gear itself, however, both as to its construction and itsmode of operation, together with additional features and advantagesthereof, will be best understood upon perusal of the following detaileddescription of certain specific embodiments with reference to theaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a fragmentary sectional viewof a motion picture projector including a variable-speed belt gear whichembodies the invention, the selector means being shown in one of itspositions;

FIG. 2 is a sectional view as seen in the direction of arrows from theline IIII of FIG. 1; and

FIG. 3 illustrates the structure of FIG. 1 but with the selector meansin another position.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIGS. 1 and2, there is shown a portion of a motion picture projector whichcomprises a housing 1. The housing 1 accommodates a prime mover hereshown as an electric motor 2 having an output shaft 3 which is separablyconnected with a first or driving two-step pulley 4. This pulleycomprises a smaller-diameter rim portion 4a, a larger-diameter rimportion 4b and a conical intermediate portion 5 which provides for agradual transition between the external surfaces of the rim portions 4aand 4b. A second or driven two-step pulley 7 is secured to the inputshaft 8 of a transmission (not shown) and comprises a smallerdiameterrim portion 7a which is coplanar with the larger-diameter rim portion4b, a larger-diameter rim portion 7b which is coplanar with thesmaller-diameter rim portion 4a and a conical intermediate portion 9which provides for a gradual transition between the external surfaces ofthe rim portions 7a and 7b. The transmission whose input shaft 8 isdriven by the pulley 7 has an output element which can drive a spool orreel for motion picture film in the projector. An endless belt 6 istrained over the pulleys 4 and 7 without resorting to a belt tighteningdevice. The diameters of the coplanar rim portions 4a, 7b and 4b, 7a areselected in such a way that the belt 6 is a tight fit when it is trainedover the rim portions 4a, 7b or over the rim portions 4b, 7a.

The housing 1 further accommodates a fixed bearing sleeve 10 for a shaft11 which carries at its outer end a speed or RPM selector knob 12. Theknob 12 has a handgrip portion 13 which is accessible at the outer sideof the housing 1. The inner portion of the selector knob 12 is providedwith two angularly spaced projections 14 and 15 best shown in FIG. 1.The housing 1 comprises two fixed stops l7 and 16 which are respectivelylocated in the path of movement of projections 14 and 15 to thusdetermine two preselected-angular positions for the knob 12. In thefirst angular position which is shown in FIG. 1, the projection 14 ofthe knob 12 abuts against the stop 17 of the housing 1. Such angularposition of the knob 12 corresponds to a first angular position of abelt shifter 23 which is turnable on the shaft 11 and has a bifurcatedend portion provided with two prongs 24, 25 which straddle the adjacentportion of the belt 6. When the shifter 23 is caused to assume the firstposition shown in FIG. 1, its prongs 24, 25 maintain the belt 6 in thecommon plane of the rim portions 4b and 7a. Consequently, when the motor2 is on to drive the pulley 4, the pulley 7 will drive the input shaft 8at a relatively high speed. FIG. 3 illustrates the knob 12 in the otherangular position in which the projection 15 of the knob abuts againstthe stop 16 of the housing 1. The belt shifter 23 is held in a secondposi tion in which its prongs 24 and 25 maintain the belt 6 in thecommon plane of the rim portions 4a and 7b. Consequently, the inputshaft 8 is rotated at a relatively low speed when the motor 2 is on torotate the pulley 4.

The knob 12 is further provided with an eccentric crank pin 18 whichextends into an eye or socket 19 at the free end of one leg of aresilient element here shown as a torsion spring 20. The free end of theother leg of the torsion spring 20 is provided with a second eye orsocket 21 which receives a pin 22 on the belt shifter 23. The pin 22 isremote from the axis around which the shifter 23 and knob 12 can turn(such axis is defined by the shaft 1 1). The belt shifter 23 is formedwith an extension 27 which bears against a fixed abutment 28 in thehousing 1 when the prongs 24 and 25 maintain the belt 6 in the commonplane of the rim portions 4a and 7b (see FIG. 3). A second fixedabutment 29 of the housing 1 is located in the path of movement of anedge face of the belt shifter 23 to arrest the latter in the positionshown in FIG. 1 in which the prongs 24, 25 maintain the belt 6 in thecommon plane of the rim portions 4b and 7a. In order to insure properguidance of the belt shifter 23 during movement from engagement with theabutment 28 into engagement with the abutment 29 or vice versa, theshifter 23 is riveted or otherwise secured to a cylindrical hub 26 whichis rotatable on the shaft 11. The prongs 24 and 25 are preferably ofcylindrical shape and constitute elongated pins or posts which areriveted or otherwise secured to that end portion of the shifter 23 whichis remote from the shaft 11.

The reference characters 30 indicate retaining rings one of which holdsthe pulley 7 against axial movement relative to the input shaft 8 andthe other of which holds the shaft 11 against axial movement in thesleeve 10. The output shaft 3 of the motor 2 has an axially parallelperipheral groove for the tip of a radially extending screw 31 whichseparably and axially adjustably secures the pulley 4 to the shaft 3.Adjustments of the pulley 4 in the axial direction of the output shaft 3are desirable in order to respectively place the rim portions 4a and 4binto common planes with the rim portions 7b and 7a.

When the belt shifter 23 dwells in the position shown in FIG. 1 in whichits edge face engages the fixed abutment 29 of the housing 1, its prongs24 and 25 maintain the belt 6 in the common plane of the rim portions 4band 7a. As mentioned above, when the motor 2 is on and the belt 6assumes the position shown in FIG. 1, the pulley 7 drives the inputshaft 8 of the transmission at a relatively high speed. In such positionof the belt shifter 23, the knob 12 dwells in its first angular positionin which its projection 14 abuts against the fixed stop 17 of thehousing 1. The spring 20 biases the knob 12 and the shifter 23 to theirillustrated positions. If the motor 2 is at a standstill and the user ofthe projector wishes to select a lower speed for the input shaft 8before the motor is started again, the knob 12 is rotated in a clockwisedirection, as viewed in FIG. 1, so that it reaches the second angularposition in which its projection 15 abuts against the fixed stop 16.During such angular movement, the knob 12 moves to and beyond adead-center intermediate position and stresses the torsion spring 20which automatically propels the knob 12 to the angular position of FIG.3 as soon as the knob moves beyond the intermediate position. Thereverse occurs when the knob 12 is rotated counterclockwise from theposition of FIG. 1. Thus, when the knob 12 reaches and moves slightlybeyond its intermediate position, the spring 20 propels the projection14 against the stop 17 so that the knob 12 reassumes the position ofFIG. 1. The spring 20 stores energy while the knob 12 moves from theposition of FIG. 1 or FIG. 3 toward the intermediate position anddissipates energy to propel the knob to the other position. If themotor2 is at a standstill while the knob 12 is being rotated from theposition of FIG. 1 to the position of FIG. 3, the prong 24 is caused tobear against the adjacent edge of the belt 6 under the action of thetorsion spring (i.e., the shifter 23 tends to rotate counterclockwise inresponse to rotation of the knob 12 in a clockwise direction, or viceversa) but is incapable of actually shifting the belt 6 from the rimportions 4b and 7a onto the rim portions 4a and 7b. However, as soon asthe motor 2 is started again, the resistance of the belt 6 to sidewisemovement decreases considerably so that the spring 20 is capable ofcausing the prong 24 to shift the belt 6 to the position shown inFIG. 1. lnversely, when the motor 2 is at a standstill, when the belt 6is trained over the rim portions 4a and 7b and the shifter 23 dwells inthe position of FIG. 3, a movement of the knob 12 from the position ofFIG. 3 to the position of FIG. 1 causes the spring 20 to urge the prongagainst the adjacent edge of the belt 6 but the belt remains in theposition of FIG. 3 as long as the motor 2 remains at a standstill.

The knob 12 can be rotated prior to or after the motor 2 is started. Thedifference is only that, when the knob 12 is rotated while the motor 2is at a standstill, the angular position of the shifter 23 changes verylittle or not at all, i.e., the position of the pin 22 remainssubstantially unchanged while the pin 18 shares the angular movement ofthe knob 12. The shifting of the belt 6 then takes place after the motor2 is started again. However, if the knob 12 is rotated while the motor 2is on, the spring 20 is free to immediately change the angular positionof the shifter 23 so that the belt 6 is moved from the rim portions 4a,7b onto the rim portions 4b, 7a or vice versa.

The pivot axis for the shifter 23 (which resembles a one-armed lever) ispreferably located in the symmetry plane between the two end positionsof the eyes 19 and 21. Consequently, the force with which the torsionspring 20 tends to turn the shifter 23 is identical for both directionsof angular movement of the shifter. It will be noted that the torsionspring 20 serves as a simple detent for the selector knob 12 as well asa means for storing energy when the knob 12 is being rotated while themotor 2 is idle. Furthermore, the spring 20 serves as a simple butreliable means for limiting the magnitude of forces which can betransmitted from the knob 12 to the shifter 23 and from the prong 24 or25 of the shifter to the belt 6. This protects the belt againstexcessive deformation, particularly against excessive stretching.

In the illustrated embodiment, the selector knob 12 is rotatabledirectly by hand. However, it is equally within the purview of theinvention to provide suitable electric, pneumatic, hydraulic or magneticremotecontrol means for changing the position of the knob 12 and forthereby changing the position of the shifter 23, either immediately orupon renewed starting of the prime mover. The cost of suchremote-control means is much less than in conventional belt gears. Anadvantage of remote-control means for moving the selector knob 12 isthat the position of the knob 12 and shifter 23 can be selected at will,i.e., these parts can be installed in such portions of the housing 1which are not accessible from without. The feature that the shifter 23is coaxial with the selector knob 12 brings about additionalsimplification in the construction of the belt gear and a furtherreduction of its space requirements. Thus, only one shaft 11 has to bemounted in the housing 1 so that the number of journals, bearings andbores can be reduced to a minimum. At least the major part of the beltgear can be assembled in advance to be thereupon inserted into andmounted in the housing 1. The number of parts in the improved belt gearis much less then in heretofore known belt gears.

The shifter 23 preferably constitutes a one-armed lever with the prongs24 and 25 at its free end. The knob 12 resembles a toggle switch. It isfurther clear that the improved belt gear can be used for driving theinput shaft 8 or another rotary element at three or more speeds. Thiscan be achieved by providing the belt gear with a suitable forcelimiting device of known design.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featureswhich fairly constitute essential characteristics of the generic andspecific aspects of my contribution to the art and, therefore, suchadaptations should and are intended to be comprehended within themeaning and range of equivalence of the claims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:

1. A variable-speed belt gear, particularly for use in motion pictureprojectors, comprising a driving pulley and a driven pulley each havingcoaxial first and second rim positions; an endless flexible belt trainedover said pulleys; and speed changing means operable to move said beltfrom said first rim portions onto said second rim portions or viceversa, including a speed selector pivotable between first and secondpositions about a predetermined axis, a belt-engaging shifter pivotableabout said predetermined axis between a first position in which saidbelt is trained over said first rim portions and a second position inwhich said belt is trained over said second rim portions, and a torsionspring for respectively urging said selector and said shifter to saidfirst and second positions in the first and second position of saidselector, said torsion spring having a first leg coupled to saidselector at a point remote from said predetermined axis and a second legcoupled to said shifter at a point remote from said predetermined axis.

2. A variable-speed belt gear, particularly for use in motion pictureprojectors, comprising a driving pulley and a driven pulley each havingcoaxial first and second rim portions; an endless flexible belt trainedover said pulleys; and speed changing means operable to move said beltfrom said first rim portions onto said second rim portions or viceversa, including a speed selector movable between first and secondpositions, a beltengaging shifter movable between a first position inwhich said belt is trained over said first rim portions and a secondposition in which said belt is trained over said second rim portions,and resilient connected between said selector and said shift means forrespectively urging said selector and said shifter to said first andsecond positions in the first and second positions of said selector, thetension of said belt being such that, when said belt is idle, saidshifter remains in its momentary position in spite of movement of saidselector from its first or second position whereby said resilient meansstores energy which suffices to move said shifter to a differentposition as soon as the belt is set in motion in response to rotation ofsaid driving pulley.

3. A belt gear as defined in claim 2, wherein said shifter includes abelt-engaging portion which is movable by said resilient meanstransversely of the direction of lengthwise movement of the adjacentportion of said belt, said selector being movable through a deadcenterposition located between said first and second positions thereof andsaid resilient means being arranged to store energy in response tomovement of said selector toward said dead-center position and tothereupon urge said selector toward said first position as soon as saidselector moves beyond said dead-center position in a direction away fromsaid second position or vice versa.

4. A belt gear as defined in claim 2, wherein said shifter and saidselector are turnable about a common axis and said resilient means isrespectively arranged to bias said shifter clockwise andcounterclockwise in response to counterclockwise and clockwise turningof said selector.

5. A belt gear as defined in claim 2, wherein said resilient meanscomprises a torsion spring.

6. A belt gear as defined in claim 2, wherein said shifter is a leverwhich is pivotable between said first and second positions thereof.

7. A belt gear as defined in claim 2, wherein said selector is a knobwhich is rotatable between said first and second positions thereof.

8. A belt gear as defined in claim 2, further comprising first andsecond abutment means for respectively holding said shifter againstmovement beyond said first and second positions thereof.

9. A belt gear as defined in claim 2, further comprising first andsecond stop means for respectively holding said selector againstmovement beyond said first and second positions thereof.

10. A belt gear as defined in claim 2, wherein the diameter of saidfirst rim portion of said first pulley exceeds the diameter of saidsecond rim portion of said first pulley and the diameter of said secondrim portion of said second pulley exceeds the diameter of said first rimportion of said second pulley, said first rim portions being located ina first common plane and said second rim portions being located in asecond common plane, and further comprising prime mover means actuatableto rotate said driving pulley and a rotary member receiving torque fromsaid driven pulley.

11. A belt gear as defined in claim 10, wherein each of said pulleyscomprises a substantially conical intermediate portion providing agradual transition between the respective first and second rim portions.

1. A variable-speed belt gear, particularly for use in motion pictureprojectors, comprising a driving pulley and a driven pulley each havingcoaxial first and second rim positions; an endless flexible belt trainedover said pulleys; and speed changing means operable to move said beltfrom said first rim portions onto said second rim portions or viceversa, including a speed selector pivotable between first and secondpositions about a predetermined axis, a belt-engaging shifter pivotableabout said predetermined axis between a first position in which saidbelt is trained over said first rim portions and a second position inwhich said belt is trained over said second rim portions, and a torsionspring for respectively urging said selector and said shifter to saidfirst and second positions in the first and second position of saidselector, said torsion spring having a first leg coupled to saidselector at a point remote from said predetermined axis and a second legcoupled to said shifter at a point remote from said predetermined axis.2. A variable-speed belt gear, particularly for use in motion pictureprojectors, comprising a driving pulley and a driven pulley each havingcoaxial first and second rim portions; an endless flexible belt trainedover said pulleys; and speed changing means operable to move said beltfrom said first rim portions onto said second rim portions or viceversa, including a speed selector movable between first and secondpositions, a belt-engaging shifter movable between a first position inwhich said belt is trained over said first rim portions and a secondposition in which said belt is trained over said second rim portions,and resilient connected between said selector and said shift means forrespectively urging said selector and said shifter to said first andsecond positions in the first and second positions of said selector, thetension of said belt being such that, when said belt is idle, saidshifter remains in its momentary position in spite of movement of saidselector from its first or second position whereby said resilient meansstores energy which suffices to move said shifter to a differentposition as soon as the belt is set in motion in response to rotation ofsaid driving pulley.
 3. A belt gear as defined in claim 2, wherein saidshifter includes a belt-engaging portion which is movable by saidresilient means transversely of the direction of lengthwise movement ofthe adjacent portion of said belt, said selector being movable through adead-center position located between said first and second positionsthereof and said resilient means being arranged to store energy inresponse to movement of said selector toward said dead-center positionand to thereupon urge said selector toward said first position as soonas said selector moves beyond said dead-center position in a directionaway from said second position or vice versa.
 4. A belt gear as definedin claim 2, wherein said shifter and said selector are turnable about acommon axis and said resilient means is respectively arranged to biassaid shifter clockwise and counterclockwise in response tocounterclockwise and clockwise turning of said selector.
 5. A belt gearas defined in claim 2, wherein said resilient means comprises a torsionspring.
 6. A belt gear as defined in claim 2, wherein said shifter is alever which is pivotable between said first and second positionsthereof.
 7. A belt gear as defined in claim 2, wherein said selector isa knob which is rotatable between said first and second positionsthereof.
 8. A belt gear as defined in claim 2, further comprising firstand second abutment means for respectively holding said shifter againstmovement beyond said first and second positions thereof.
 9. A belt gearas defined in claim 2, further comprising first and second stop meansfor respectively holding said selector against movement beyond saidfirst and second positions thereof.
 10. A belt gear as defined in claim2, wherein the diameter of said first rim portion of said first pulleyexceeds the diameter of said second rim portion of said first pulley andthe diameter of said second rim portion of said second pulley exceedsthe diameter of said first rim portion of said second pulley, said firstrim portions being located in a first common plane and said second rimportions being located in a second common plane, and further comprisingprime mover means actuatable to rotate said driving pulley and a rotarymember receiving torque from said driven pulley.
 11. A belt gear asdefined in claim 10, wherein each of said pulleys comprises asubstantially conical intermediate portion providing a gradualtransition between the respective first and second rim portions.